Why video appears in lower left corner when offset is checked in strip transform parameters?
Offset and Crop are working in pretty odd ways: https://developer.blender.org/T74323
The best way to deal with them is to use the Transform Tools add-on(until they're fixed):
https://github.com/doakey3/VSE_Transform_Tools
After installing, you'll find the Transform options in the Toolbar in the left side of the Preview:

(Btw. in order to get Transform Tools working, you'll need to update Blender to 2.83)
In the latest beta of 2.90 the bug you mention may have been fixed.